Laser-based dermatological treatment gained high popularity in aesthetic medicine and laser hair removal is the most popular medical cosmetic application. U.S. Pat. No. 5,595,568 describes a method of permanent hair removal using lasers of different wavelengths, the contents of which patent are herein incorporated by reference in their entirety. This method of hair removal is based on principles of selective photothermolisis, including the following:                a. Light should penetrate into the skin deep enough to reach treated target; and        b. Light absorption in the target should be higher than in surrounding tissue.        
The most suitable wavelengths for hair removal are in the range of 600 nm to 1100 nm, with longer wavelengths in the range being safer for darker skin and shorter wavelengths being more effective for hair follicle destruction, but less safe for darker skin. Wavelengths longer than 1200 nm are generally not used for hair removal because of absorption by water. These longer wavelengths can be used for unselective heating of skin and fat. Wavelengths shorter than 600 nm have achieve minimal penetration depth, on the order of 1.5 mm or less, and are therefore not effective for hair removal.
The most popular lasers used for hair removal are:                a. Ruby lasers with a wavelength of 694 nm. Lasers with this wavelength have very high absorption in melanin, which is the main chromophor in the hair and the epidermis, but the penetration depth of this wavelength is not very high. These lasers are considered to be very hard on the epidermis, and their use has declined significantly in recent years.        b. Alexandrite lasers with a wavelength of 755 nm are very effective for hair removal, but less safe for darker skin.        c. Laser diodes with wavelengths of 800 nm-820 nm, which are considered safe and effective for most of the skin types, but can be less effective for removal of fine and lighter hairs.        d. Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers with a wavelength of 1064 nm. This laser has an excellent penetration depth, but not very high absorption in the melanin. As a result it is very safe for all skin types, but effective for removal of black and coarse hairs only.        
There are no optimal wavelengths between 820 nm and 1064 nm because of a broad peak of water absorption which limits light penetration depth.
Recently, laser diode technology has developed dramatically and effective lasers with a wide range of different wavelengths are now available. U.S. Patent Application Publication No. 2009/0254068 describes a device and method of hair removal using a laser diode laser with a wavelength 750 nm to 1500 nm, applied in pulses with a high repetition rate, the contents of which application are herein incorporated by reference in their entirety. Laser diodes are also used for treatment of vascular lesions, collagen remodeling, fat destruction and other therapeutic applications and treatments.
Wavelengths of laser diodes can be selected during the design of treatment devices, but a single laser wavelength does not always provide optimal treatment and combination of two or more laser wavelengths can be preferable. For example, U.S. Patent Application Publication No. 2012/0165800 describes a device with a few single laser diode emitters, which emitters may have different wavelengths. However, single laser diodes emitters are not powerful enough for the majority medical and cosmetic applications where selective coagulation of tissue is required.